Unveiling the Promoting Mechanism of H2 Activation on CuFeOx Catalyst for Low-Temperature CO Oxidation

The effect of H2 activation on the performance of CuFeOx catalyst for low-temperature CO oxidation was investigated. The characterizations of XRD, XPS, H2-TPR, O2-TPD, and in situ DRIFTS were employed to establish the relationship between physicochemical property and catalytic activity. The results showed that the CuFeOx catalyst activated with H2 at 100 °C displayed higher performance, which achieved 99.6% CO conversion at 175 °C. In addition, the H2 activation promoted the generation of Fe2+ species, and more oxygen vacancy could be formation with higher concentration of Oα species, which improved the migration rate of oxygen species in the reaction process. Furthermore, the reducibility of the catalyst was enhanced significantly, which increased the low-temperature activity. Moreover, the in situ DRIFTS experiments revealed that the reaction pathway of CO oxidation followed MvK mechanism at low temperature (<175 °C), and both MvK and L-H mechanism was involved at high temperature. The Cu+-CO and carbonate species were the main reactive intermediates, and the H2 activation increased the concentration of Cu+ species and accelerated the decomposition carbonate species, thus improving the catalytic performance effectively.


Introduction
The incomplete combustion of fossil fuels results in industrial flue gases containing certain concentrations of CO.For example, the low-temperature flue gas (80-180 • C) generated during the iron ore sintering process contains 0.5-2% CO [1,2].The ability of CO to bind to hemoglobin is stronger than oxygen.The direct release of CO is detrimental to human health and causes various environmental issues, including the degradation of the ozone layer [3][4][5].Catalytic oxidation could efficiently oxidize CO into nontoxic CO 2 with minimal energy consumption, which is considered to be the most promising CO elimination technology at present [6,7].Au, Pt, and other precious metal catalysts exhibit high catalytic activity [8].Nevertheless, the elevated expense associated with precious metal catalysts poses a constraint on the widespread deployment of CO oxidation in largescale industrial flue gas treatment.It is necessary to develop a catalytic material with low price and excellent performance for CO oxidation.
The Cu-based catalysts display a great development prospect in CO oxidation due to the unique CO adsorption and activation property [9,10].However, the pure CuO catalyst has poor activity at low temperature.The catalyst of CuO combined with CeO 2 , Fe 2 O 3, Co 3 O 4 , and MnO 2 shows high catalytic performance at low temperatures [11][12][13][14][15]. Notably, the CuFeO x catalysts display exceptional performance in CO oxidation, attributed to the remarkable oxygen storage capacity and Fe 2+ /Fe 3+ redox cycle inherent in Fe 2 O 3 [16][17][18].This suggests a potential for CuFeO x catalysts to supplant precious metal catalysts in CO removal from industrial flue gas.According to our previous research [19,20], the structure and oxidation state of the active components have significant effects on the catalytic performance.Previous studies have shown that the pretreatment and activation could adjust the structure and the content of oxidation states of active components of catalysts [21][22][23], which could effectively improve the catalytic performance.Wang et al. [21] reported that the H 2 pretreated Pd/Fe 2 O 3 catalyst exhibited higher catalytic activity due to the higher concentration of Fe 2+ species and stronger reducibility.Wang et al. [22] explored the impact of H 2 activation on the catalytic activity of the CuCeO x catalyst.The H 2 pretreatment increased the amount of highly dispersed CuO x and oxygen vacancy, which enhanced the activity of CO oxidation.This showed us that the performance of the CuFeO x catalyst may be improved by H 2 pretreatment.However, the effect of H 2 pretreatment on the performance of the CuFeO x catalyst has seldom been explored in detail.
In this work, the CuFeO x catalyst was prepared and activated with H 2 at different temperatures to investigate the influence of pretreatment on the performance.In addition, the XRD, BET, XPS, H 2 -TPR, and O 2 -TPD were characterized to establish the relationship between structure property and catalytic performance.Furthermore, the reaction mechanism was revealed by in situ DRIFTS studies.This work may provide a novel strategy for the improvement of performance of the CuFeO x catalyst.

Catalytic Performance
The activity of catalysts was measured and is displayed in Figure 1a.It was observed that the CO conversion increased with increasing temperature for all catalysts.As for pure Fe 2 O 3 , the CO conversion was only 41.7% at 250 • C. As expected, the activity was improved after the addition of Cu species, and 100% CO conversion could be achieved at 250 • C for the CuFeO x catalyst.In addition, the activity of CuFeO x and H 2 -activated catalysts was insignificantly different at low temperature, while the activities of CuFeO x -100 and CuFeO x -150 catalysts were higher than CuFeO x catalysts after 75 • C. The CuFeO x -100 catalyst displayed the highest activity with 99.6% CO conversion at 175 • C, which indicated that the appropriate activation temperature could effectively increase the catalytic performance.However, it could be noted that the CO conversion of the CuFeO x -200 catalyst was obviously lower than the CuFeO x catalyst, which may be due to the over-reduction decreasing the active species.The service life is another important factor in evaluating catalyst applications.The stability of the CuFeO x -100 catalyst was measured and is shown in Figure 1b.The reaction continued for 82 h at 175 • C, keeping the composition and airspeed of the gas mixture unchanged.It can be seen that the CO conversion almost did not decrease after 82 h, and displayed excellent stability.
Molecules 2024, 29, x FOR PEER REVIEW 2 of 14 removal from industrial flue gas.According to our previous research [19,20], the structure and oxidation state of the active components have significant effects on the catalytic performance.Previous studies have shown that the pretreatment and activation could adjust the structure and the content of oxidation states of active components of catalysts [21][22][23], which could effectively improve the catalytic performance.Wang et al. [21] reported that the H2 pretreated Pd/Fe2O3 catalyst exhibited higher catalytic activity due to the higher concentration of Fe 2+ species and stronger reducibility.Wang et al. [22] explored the impact of H2 activation on the catalytic activity of the CuCeOx catalyst.The H2 pretreatment increased the amount of highly dispersed CuOx and oxygen vacancy, which enhanced the activity of CO oxidation.This showed us that the performance of the CuFeOx catalyst may be improved by H2 pretreatment.However, the effect of H2 pretreatment on the performance of the CuFeOx catalyst has seldom been explored in detail.
In this work, the CuFeOx catalyst was prepared and activated with H2 at different temperatures to investigate the influence of pretreatment on the performance.In addition, the XRD, BET, XPS, H2-TPR, and O2-TPD were characterized to establish the relationship between structure property and catalytic performance.Furthermore, the reaction mechanism was revealed by in situ DRIFTS studies.This work may provide a novel strategy for the improvement of performance of the CuFeOx catalyst.

Catalytic Performance
The activity of catalysts was measured and is displayed in Figure 1a.It was observed that the CO conversion increased with increasing temperature for all catalysts.As for pure Fe2O3, the CO conversion was only 41.7% at 250 °C.As expected, the activity was improved after the addition of Cu species, and 100% CO conversion could be achieved at 250 °C for the CuFeOx catalyst.In addition, the activity of CuFeOx and H2-activated catalysts was insignificantly different at low temperature, while the activities of CuFeOx-100 and CuFeOx-150 catalysts were higher than CuFeOx catalysts after 75 °C.The CuFeOx-100 catalyst displayed the highest activity with 99.6% CO conversion at 175 °C, which indicated that the appropriate activation temperature could effectively increase the catalytic performance.However, it could be noted that the CO conversion of the CuFeOx-200 catalyst was obviously lower than the CuFeOx catalyst, which may be due to the over-reduction decreasing the active species.The service life is another important factor in evaluating catalyst applications.The stability of the CuFeOx-100 catalyst was measured and is shown in Figure 1b.The reaction continued for 82 h at 175 °C, keeping the composition and airspeed of the gas mixture unchanged.It can be seen that the CO conversion almost did not decrease after 82 h, and displayed excellent stability.

Structural and Textural Properties
The crystal structure of the catalysts was investigated with XRD and is depicted in Figure 2. The pure Fe 2 O 3 displayed the diffraction peaks at 24.2 • , 33.2   , 64.0 • , 71.9 • , and 75.5 • , which corresponded to the crystallized α-Fe 2 O 3 for (012), ( 104), ( 110), ( 113), (024), ( 116), ( 122), ( 214), (300), (101), and (220), respectively.The diffraction peaks of pure Fe 2 O 3 presented with high crystallinity.As for the CuFeO x catalyst, new peaks attributed to CuO at 38.8 • , 48.6 • , and 61.6 • appeared.In addition, the peak intensity of Fe 2 O 3 decreased significantly compared with pure Fe 2 O 3 , indicating the formation of a strong interaction between Fe and Cu species, as a result of the smaller particle size and reduced crystallinity.For CuFeO x -100 and CuFeO x -150 catalysts, the peaks at 48.6 • and 61.6 • weakened and even disappeared, possibly attributable to the reduction of CuO to Cu 2 O and/or metallic Cu.The peaks assigned to metallic copper at 43.4 • and 50.5 • could be found on the CuFeO x -200 catalyst, which confirmed that part of the CuO species was reduced to metallic Cu species due to the H 2 activation.Interestingly, the peaks of Fe 2 O 3 at 35.7 • on the H 2 -activated catalysts shifted to a lower 2θ degree compared to the CuFeO x catalyst.This demonstrated that the H 2 activation promoted the formation of Fe 2+ species and resulted in the lattice expansion, which suggested that the Fe 2 O 3 also participated in the reduction process.In addition, no obvious diffraction peaks related to the CuFeOx phase were found.

Structural and Textural Properties
The crystal structure of the catalysts was investigated with XRD and is depicted in Figure 2. The pure Fe2O3 displayed the diffraction peaks at 24.2°, 33.2°, 35.6°, 40.9°, 49.5°, 54.1°, 57.6°, 62.4°, 64.0°, 71.9°, and 75.5°, which corresponded to the crystallized α-Fe2O3 for (012), (104), (110), ( 113), (024), ( 116), (122), (214), (300), (101), and (220), respectively.The diffraction peaks of pure Fe2O3 presented with high crystallinity.As for the CuFeOx catalyst, new peaks attributed to CuO at 38.8°, 48.6°, and 61.6° appeared.In addition, the peak intensity of Fe2O3 decreased significantly compared with pure Fe2O3, indicating the formation of a strong interaction between Fe and Cu species, as a result of the smaller particle size and reduced crystallinity.For CuFeOx-100 and CuFeOx-150 catalysts, the peaks at 48.6° and 61.6° weakened and even disappeared, possibly attributable to the reduction of CuO to Cu2O and/or metallic Cu.The peaks assigned to metallic copper at 43.4° and 50.5° could be found on the CuFeOx-200 catalyst, which confirmed that part of the CuO species was reduced to metallic Cu species due to the H2 activation.Interestingly, the peaks of Fe2O3 at 35.7° on the H2-activated catalysts shifted to a lower 2θ degree compared to the CuFeOx catalyst.This demonstrated that the H2 activation promoted the formation of Fe 2+ species and resulted in the lattice expansion, which suggested that the Fe2O3 also participated in the reduction process.In addition, no obvious diffraction peaks related to the CuFeOx phase were found.The N2 adsorption-desorption isotherms are displayed in Figure S1, and the physical property of the catalysts is shown in Table 1.As shown in Figure S1, all catalysts presented type IV isotherms and H3 type hysteresis loops, which suggests the existence of mesopores.The mesopores were conducive to the dispersion of active particles and the diffusion of reactive gases.The specific surface area, average pore diameter, and total pore volume of pure Fe2O3 was 9.85 m 2 /g, 14.96 nm, and 0.037 cm 3 /g, respectively.After the addition of the Cu species, the specific surface area and total pore volume of the CuFeOx catalyst increased slightly, while the average pore diameter was reduced.This suggests that the strong interaction among Fe and Cu species hindered the growth of particles, thereby enhancing the dispersion of active components, and more active sites could be provided for CO oxidation.In addition, the difference in specific surface area, average pore diameter, and total pore volume between CuFeOx and CuFeOx-100 catalysts could be negligible, which demonstrates that the influence of H2 activation on the surface structure of the catalyst was less.The CuFeOx and CuFeOx-100 catalysts had similar surface structure, while the activity of the CuFeOx-100 catalyst was obviously higher than the CuFeOx catalyst.This suggests that the surface physical properties were not the The N 2 adsorption-desorption isotherms are displayed in Figure S1, and the physical property of the catalysts is shown in Table 1.As shown in Figure S1, all catalysts presented type IV isotherms and H3 type hysteresis loops, which suggests the existence of mesopores.The mesopores were conducive to the dispersion of active particles and the diffusion of reactive gases.The specific surface area, average pore diameter, and total pore volume of pure Fe 2 O 3 was 9.85 m 2 /g, 14.96 nm, and 0.037 cm 3 /g, respectively.After the addition of the Cu species, the specific surface area and total pore volume of the CuFeO x catalyst increased slightly, while the average pore diameter was reduced.This suggests that the strong interaction among Fe and Cu species hindered the growth of particles, thereby enhancing the dispersion of active components, and more active sites could be provided for CO oxidation.In addition, the difference in specific surface area, average pore diameter, and total pore volume between CuFeO x and CuFeO x -100 catalysts could be negligible, which demonstrates that the influence of H 2 activation on the surface structure of the catalyst was less.The CuFeO x and CuFeO x -100 catalysts had similar surface structure, while the activity of the CuFeO x -100 catalyst was obviously higher than the CuFeO x catalyst.This suggests that the surface physical properties were not the determining factor for the increased activity of the CuFeO x -100 catalyst.The morphological features of the catalysts were investigated with SEM, as displayed in Figure S2.The catalysts existed in irregular flakes and particles of various sizes.The surface morphology of the catalysts had no obvious change after H 2 activation.

Surface Chemical States
The effect of H 2 activation on surface chemical composition on the catalysts was determined with XPS, as depicted in Figure 3.In addition, the atomic percentages of elements were calculated from the areas of the fitted peaks and were presented in Table 2.The Cu 2p of catalysts are depicted in Figure 3a, which could be separated into Cu 2p 1/2 and Cu 2p 3/2 , respectively [24].The two peaks of Cu 2p 3/2 located at 933.4 and 934.8 eV corresponded to Cu + and Cu 2+ species, respectively [1,25].In addition, the shake-up satellite peak at 942.3 eV was assigned to CuO.According to previous studies [9,26,27], the Cu + species was the main adsorption and activation sites of CO, playing a crucial role in the CO oxidation process.It could be noted that the proportion of the CuFeO x catalyst was 10.2%, which was much lower than that of 17.5% on the CuFeO x -100 catalyst.This confirmed that the H 2 activation could promote the generation of Cu + species, thus increasing the CO conversion.
determining factor for the increased activity of the CuFeOx-100 catalyst.The morpholo cal features of the catalysts were investigated with SEM, as displayed in Figure S2.T catalysts existed in irregular flakes and particles of various sizes.The surface morpholo of the catalysts had no obvious change after H2 activation.

Surface Chemical States
The effect of H2 activation on surface chemical composition on the catalysts was d termined with XPS, as depicted in Figure 3.In addition, the atomic percentages of e ments were calculated from the areas of the fitted peaks and were presented in Table The Cu 2p of catalysts are depicted in Figure 3a, which could be separated into Cu 2p and Cu 2p3/2, respectively [24].The two peaks of Cu 2p3/2 located at 933.4 and 934.8 corresponded to Cu + and Cu 2+ species, respectively [1,25].In addition, the shake-up sat lite peak at 942.3 eV was assigned to CuO.According to previous studies [9,26,27], the C species was the main adsorption and activation sites of CO, playing a crucial role in t CO oxidation process.It could be noted that the proportion of the CuFeOx catalyst w 10.2%, which was much lower than that of 17.5% on the CuFeOx-100 catalyst.This co firmed that the H2 activation could promote the generation of Cu + species, thus increasi the CO conversion.The O 1s spectra are depicted in Figure 3b.There were two fitted peaks at 529.5 a 531.3 eV, which were attributed to the surface lattice oxygen species (O 2− ) and chemisorb oxygen species (O2-), marked as Oβ and Oα, respectively [28].Generally, the Oα species w formed on oxygen vacancy with higher mobility than Oβ species [4,29,30].In addition was associated with charge unbalance, which was beneficial to the improvement of red cycle, and an increase in low-temperature activity.It was found that the concentration Oα species increased from 30.1% for the CuFeOx catalyst to 39.6% for the CuFeOx-100 c alyst, which implied that the H2 activation could accelerate the generation of oxygen v cancy.In addition, it was observed that the peak Oβ and Oα on the CuFeOx-100 cataly shifted towards lower binding energy compared with the CuFeOx catalyst.This may due to the H2 activation weakening the electron-withdrawing ability of Cu and/or  The O 1s spectra are depicted in Figure 3b.There were two fitted peaks at 529.5 and 531.3 eV, which were attributed to the surface lattice oxygen species (O 2− ) and chemisorbed oxygen species (O 2-), marked as O β and O α , respectively [28].Generally, the O α species was formed on oxygen vacancy with higher mobility than O β species [4,29,30].In addition, it was associated with charge unbalance, which was beneficial to the improvement of redox cycle, and an increase in low-temperature activity.It was found that the concentration of O α species increased from 30.1% for the CuFeO x catalyst to 39.6% for the CuFeO x -100 catalyst, which implied that the H 2 activation could accelerate the generation of oxygen vacancy.In addition, it was observed that the peak O β and O α on the CuFeO x -100 catalyst shifted towards lower binding energy compared with the CuFeO x catalyst.This may be due to the H 2 activation weakening the electron-withdrawing ability of Cu and/or Fe species and increasing the surrounding electron cloud density, which resulted in the decrease in binding energy.
For Fe 2p in Figure 3c, two main peaks at 710.8 and 724.0 eV for Fe 2p 3/2 and Fe 2p 1/2 were found on the CuFeO x catalyst [31,32], and a satellite peak at 719.4 eV for Fe 3+ species was observed [33,34].As for the CuFeO x -100 catalyst, peaks at 709.3 and 716.8 eV associated with Fe 2+ species appeared, which confirmed that the H 2 activation promoted the Fe 3+ species reduced to Fe 2+ species, in agreement with the XRD results.Previous studies had illustrated that the O 2 could be dissociated to atomic O without a barrier by Fe 2+ species [35,36].The activity of CO oxidation was well correlated with the existence and concentration of Fe 2+ species on the catalyst.Additionally, the coexistence of Fe 2+ and Fe 3+ species could establish the redox equilibrium of Cu + + Fe 3+ ↔Cu 2+ + Fe 2+ on the catalyst surface, which could facilitate the electron transfer between active components to improve the redox cycle.In summary, the H 2 activation generated Fe 2+ species, which effectively increased the activity of the CuFeO x catalyst.

Redox Properties
The H 2 -TPR was performed to measure the reducibility of catalysts and is shown in Figure 4a.As for the CuFeO x catalyst, the strong peak at 261.0 • C could be assigned to the coreduction of CuO to metallic Cu and Fe 2 O 3 to Fe 3 O 4 , while the broad peaks in the range of 400 to 800 • C belonged to the sequential reduction of Fe 3 O 4 to FeO and then to Fe.Compared to the pure Fe 2 O 3 catalyst [12], the reduction temperature of Fe 2 O 3 to Fe 3 O 4 decreased to below 300 • C, indicating that the synergistic effect between Cu and Fe species significantly improved the redox capacity.That is, the Cu species as the active sites facilitated the overflow of H 2 to Fe 2 O 3 , thus accelerating the reduction to Fe 3 O 4 at low temperature.Additionally, a shoulder peak appeared on the H 2 -activated catalyst at lower temperature, which may be ascribed to the reduction of CuO or/and Cu 2 O to metallic Cu.This further confirmed that the H 2 activation promoted the generation of more Cu 2 O.Moreover, the initial reduction temperature on the CuFeO x catalyst was 120 • C, while it shifted to 98 • C on the CuFeO x -100 catalyst.The improvement in initial redox property could effectively increase the catalytic activity at low temperature.The Fe species reduced at 400-800 • C had almost no catalytic activity, and activity tests were performed at low temperatures, so it was not involved in the reaction processes.The H 2 consumption corresponding to the reduction peak below 400 • C was determined based on its integrated area, as depicted in Figure 4b.The total H 2 consumption on the CuFeO x catalyst slightly decreased from 1645.2 µmol/g to 1639.6 and 1555.7 µmol/g on the CuFeO x -100 and CuFeO x -150 catalysts, respectively, while it significantly decreased to 1390.6 µmol/g on the CuFeO x -200 catalyst.The H 2 activation at high temperature resulted in part of the active metal oxides being reduced to a metallic state, which may be the main reason for the decline in CO conversion of the CuFeO x -200 catalyst.

Temperature-Programmed Studies
The effect of H2 activation on the desorption of oxygen species was studied by O2-TPD, as exhibited in Figure 5.In Figure 5a, the desorption curves are categorized into three temperature regions [2,37]: (1) The surface chemisorbed oxygen (<200 °C), which is associated with the oxygen vacancy; (2) surface lattice oxygen (200-500 °C); and (3) bulk lattice oxygen (>500 °C).It can be noted that the desorption of surface chemisorbed oxygen on the H2-activated catalysts shifted to a lower-temperature position, which implies that its transfer ability was enhanced, and there was favorability for low-temperature CO oxidation.Additionally, the concentration of oxygen species was calculated by the peak area, as displayed in Figure 5b.Compared with the CuFeOx catalyst (15.0%), the proportion of surface chemisorbed oxygen species on the CuFeOx-100 (21.4%) and CuFeOx-150 (17.4%) catalysts obviously increased, which confirms that the H2 activation at a suitable temperature could produce more defects on the catalyst.In addition, the bulk lattice oxygen decreased from 19.2% on the CuFeOx catalyst to 16.5% and 13.3% on the CuFeOx-100 and CuFeOx-150 catalysts, respectively.This implied an enhancement in the migration of oxygen species from deeper layers to the surface, thereby improving catalytic performance effectively.It can be noted that the percentage of surface chemisorbed oxygen species declined on the CuFeOx-200 catalyst (12.7%) compared with the CuFeOx catalyst, which may be due to the overreduction of oxides to the metallic state, which resulted in the decrease in oxygen vacancy.

Temperature-Programmed Studies
The effect of H 2 activation on the desorption of oxygen species was studied by O 2 -TPD, as exhibited in Figure 5.In Figure 5a, the desorption curves are categorized into three temperature regions [2,37]: (1) The surface chemisorbed oxygen (<200 • C), which is associated with the oxygen vacancy; (2) surface lattice oxygen (200-500 • C); and (3) bulk lattice oxygen (>500 • C).It can be noted that the desorption of surface chemisorbed oxygen on the H 2 -activated catalysts shifted to a lower-temperature position, which implies that its transfer ability was enhanced, and there was favorability for low-temperature CO oxidation.Additionally, the concentration of oxygen species was calculated by the peak area, as displayed in Figure 5b.Compared with the CuFeO x catalyst (15.0%), the proportion of surface chemisorbed oxygen species on the CuFeO x -100 (21.4%) and CuFeO x -150 (17.4%) catalysts obviously increased, which confirms that the H 2 activation at a suitable temperature could produce more defects on the catalyst.In addition, the bulk lattice oxygen decreased from 19.2% on the CuFeO x catalyst to 16.5% and 13.3% on the CuFeO x -100 and CuFeO x -150 catalysts, respectively.This implied an enhancement in the migration of oxygen species from deeper layers to the surface, thereby improving catalytic performance effectively.It can be noted that the percentage of surface chemisorbed oxygen species declined on the CuFeO x -200 catalyst (12.7%) compared with the CuFeO x catalyst, which may be due to the overreduction of oxides to the metallic state, which resulted in the decrease in oxygen vacancy.

Reaction Intermediates Analysis
2.6.1.In Situ DRIFT Spectra of CO Adsorption, Ar and O2 Purging To investigate the active intermediates involved in the CO oxidation reaction, in situ DRIFT was performed.Firstly, the samples were purged in 1% CO/Ar from 25 to 175 °C with temperature rate of 5 min/°C, then switched with pure Ar for 5 min, and finally injected pure 15% O2/Ar with 5 min.As seen in Figure 6(a1) for the CuFeOx catalyst, the peaks at 2110 and 2172 cm −1 associated with Cu + -CO species could be found [38,39], and the intensity of Cu + -carbonyl species (Cu + -CO) strengthened with the increase in temperature initially and then decreased (Figure 6(c1)), which indicates that the Cu + species was the main active site for CO adsorption and activation.Additionally, the peaks assigned to carbonate species (1014, 1103, and 1361 cm −1 ) [40], bicarbonate species (838 and 1245 cm −1 ), [41] and monodentate carbonates (1441 cm −1 ) [42] also declined with increasing temperature and were then enhanced, while the peak at 1605 cm −1 , ascribed to bicarbonate species, decreased [43].This illustrates that the carbonate species were the active intermediates in the CO oxidation process.In addition, the weak peaks at 2320 and 2356 cm −1 belonging to CO2 were found even at 25 °C [40], which proves that the CO oxidation can be conducted at ambient temperature, consistent with the results of activity testing.The catalysts were pretreated with pure Ar for 60 min at 300 °C, and the surface adsorbed oxygen was removed completely.Therefore, the CO2 originated from the reaction between the Cu + -CO species and surface lattice oxygen, and the process followed the MvK mechanism.As described in Figure 6(a2), the intensity of Cu + -CO species declined after purging with Ar at 175 °C.When O2 was introduced (Figure 6(a3)), the intensity of the Cu + -CO species declined and the peak at 1605 cm −1 (bicarbonate species) was enhanced, which illustrates that the consumed active oxygen species can be replenished with gaseous phase O2.As for the CuFeOx-100 catalyst in Figure 6(b1), the type of species was similar to the CuFeOx catalyst, while the intensity of the Cu + -CO and carbonate species was stronger (Figure 6(c1,c2)).This demonstrates that the quantities of Cu + and active oxygen species on the CuFeOx-100 catalyst were higher than those of the CuFeOx catalyst, in agreement with the XPS and O2-  To investigate the active intermediates involved in the CO oxidation reaction, in situ DRIFT was performed.Firstly, the samples were purged in 1% CO/Ar from 25 to 175 • C with temperature rate of 5 min/ • C, then switched with pure Ar for 5 min, and finally injected pure 15% O 2 /Ar with 5 min.As seen in Figure 6(a 1 ) for the CuFeO x catalyst, the peaks at 2110 and 2172 cm −1 associated with Cu + -CO species could be found [38,39], and the intensity of Cu + -carbonyl species (Cu + -CO) strengthened with the increase in temperature initially and then decreased (Figure 6(c 1 )), which indicates that the Cu + species was the main active site for CO adsorption and activation.Additionally, the peaks assigned to carbonate species (1014, 1103, and 1361 cm −1 ) [40], bicarbonate species (838 and 1245 cm −1 ), [41] and monodentate carbonates (1441 cm −1 ) [42] also declined with increasing temperature and were then enhanced, while the peak at 1605 cm −1 , ascribed to bicarbonate species, decreased [43].This illustrates that the carbonate species were the active intermediates in the CO oxidation process.In addition, the weak peaks at 2320 and 2356 cm −1 belonging to CO 2 were found even at 25 • C [40], which proves that the CO oxidation can be conducted at ambient temperature, consistent with the results of activity testing.The catalysts were pretreated with pure Ar for 60 min at 300 • C, and the surface adsorbed oxygen was removed completely.Therefore, the CO 2 originated from the reaction between the Cu + -CO species and surface lattice oxygen, and the process followed the MvK mechanism.As described in Figure 6(a 2 ), the intensity of Cu + -CO species declined after purging with Ar at 175 • C. When O 2 was introduced (Figure 6(a 3 )), the intensity of the Cu + -CO species declined and the peak at 1605 cm −1 (bicarbonate species) was enhanced, which illustrates that the consumed active oxygen species can be replenished with gaseous phase O 2 .As for the CuFeO x -100 catalyst in Figure 6(b 1 ), the type of species was similar to the CuFeO x catalyst, while the intensity of the Cu + -CO and carbonate species was stronger (Figure 6(c 1 ,c 2 )).This demonstrates that the quantities of Cu + and active oxygen species on the CuFeO x -100 catalyst were higher than those of the CuFeO x catalyst, in agreement with the XPS and O 2 -TPD analysis.Furthermore, the intensity of Cu + -CO species began to reduce after 125 • C both for CuFeO x and CuFeO x -100 catalyst (Figure 6(c 1 ,c 2 )), which suggests that the reaction had entered a phase of rapid progress.It was found that the reduction range of intensity for Cu + -CO species on the CuFeO x -100 catalyst was larger than that on the CuFeO x catalyst after 125 • C (Figure 6(c 1 )), which indicates that the Cu + -CO species was more active.
Molecules 2024, 29, x FOR PEER REVIEW 8 of 14 TPD analysis.Furthermore, the intensity of Cu + -CO species began to reduce after 125 °C both for CuFeOx and CuFeOx-100 catalyst (Figure 6(c1,c2)), which suggests that the reaction had entered a phase of rapid progress.It was found that the reduction range of intensity for Cu + -CO species on the CuFeOx-100 catalyst was larger than that on the CuFeOx catalyst after 125 °C (Figure 6(c1)), which indicates that the Cu + -CO species was more active.

CO+O 2 Coadsorption
The in situ DRFITS of CO+O 2 coadsorption is displayed in Figure 7.As depicted in Figure 7a,b, the species and variation trend of intensity with temperature was similar in CO atmosphere both for CuFeO x and CuFeO x -100 catalysts.As illustrated in Figure 7c, the intensity of the Cu + -CO species on the CuFeO x and CuFeO x -100 catalysts was decreased to 100 and 75 • C compared with CO atmosphere, respectively, which proved that the O 2 -riched atmosphere was conducive to the CO oxidation.In addition, the Cu + -CO species on the CuFeO x -100 catalyst was stronger than the CuFeO x catalyst at 25 • C and lower at higher temperature, which further confirms that the Cu + -CO species with H 2 activation was more active.As displayed in Figure 7d, the carbonate species both on CuFeO x and CuFeO x -100 catalysts was lower than in CO atmosphere, which implies that the presence of O 2 can promote the decomposition of carbonate species.In contrast to the CO atmosphere (Figure 6(c 2 )), the strength of carbonate species on the CuFeO x -100 catalyst was higher than that on the CuFeO x catalyst (Figure 7d), indicating that carbonate species were more easily decomposed on the CuFeO x -100 catalyst in reaction to atmosphere.Interestingly, it was found that the intensity of the carbonate species started to decrease and was accompanied by the generation of CO 2 sharply after 175 • C, which suggests that the CO oxidation process was conducted through the L-H mechanism.
Molecules 2024, 29, x FOR PEER REVIEW 9 of 14 The in situ DRFITS of CO+O2 coadsorption is displayed in Figure 7.As depicted in Figure 7a,b, the species and variation trend of intensity with temperature was similar in CO atmosphere both for CuFeOx and CuFeOx-100 catalysts.As illustrated in Figure 7c, the intensity of the Cu + -CO species on the CuFeOx and CuFeOx-100 catalysts was decreased to 100 and 75 °C compared with CO atmosphere, respectively, which proved that the O2riched atmosphere was conducive to the CO oxidation.In addition, the Cu + -CO species on the CuFeOx-100 catalyst was stronger than the CuFeOx catalyst at 25 °C and lower at higher temperature, which further confirms that the Cu + -CO species with H2 activation was more active.As displayed in Figure 7d, the carbonate species both on CuFeOx and CuFeOx-100 catalysts was lower than in CO atmosphere, which implies that the presence of O2 can promote the decomposition of carbonate species.In contrast to the CO atmosphere (Figure 6(c2)), the strength of carbonate species on the CuFeOx-100 catalyst was higher than that on the CuFeOx catalyst (Figure 7d), indicating that carbonate species were more easily decomposed on the CuFeOx-100 catalyst in reaction to atmosphere.Interestingly, it was found that the intensity of the carbonate species started to decrease and was accompanied by the generation of CO2 sharply after 175 °C, which suggests that the CO oxidation process was conducted through the L-H mechanism.Behavior of Cu + −CO species at 2110 cm −1 (c) and carbonate species at 1361 cm −1 (d).( ♣ bicarbonate species, ♦ carbonate species, ♥ monodentate carbonates, ▲Cu + -CO species, CO2).

Discussion
Based on the above results, the H 2 activation with appropriate temperature can efficiently increase the activity of CuFeO x catalyst for CO oxidation (Figure 1a).XRD analysis (Figure 2) showed that the pure Fe 2 O 3 had good crystallization and large grain size, and the addition of Cu species can decrease the crystallization.In addition, some peaks attributed to CuO disappeared with the H 2 pretreatment at 100 and 150 • C, confirming that the CuO was reduced to Cu 2 O.After pretreatment at 200 • C, the metal Cu was produced.The specific surface area increased slightly with the introduction of Cu species (Table 1), which could promote the dispersion of active components and adsorption of reaction gas.Following H 2 activation, there was minimal change observed in the specific surface area, average pore diameter, and pore volumes of the catalysts.This suggests that the surface physical structure may not be the primary factor contributing to the improvement in catalytic performance.Furthermore, the XPS results (Figure 4) illustrate that the percentage of Cu + species increased on the H 2 -activated catalyst, which could offer more active sites for CO, and more O α species were produced with the H 2 pretreatment, which proved that more oxygen vacancy was formed.This could facilitate the activation of oxygen species and the circulation of the reaction.In addition, the Fe 2+ species was generated, in agreement with the XRD results, which was beneficial to the dissociation of O 2 to oxygen atoms.Moreover, the H 2 -TPR analysis indicated that the moderate H 2 activation could enhance the reducibility of the CuFeO x catalyst, which could increase the catalytic activity for CO oxidation.However, the H 2 activation at high temperature led to the decrease in H 2 consumption and redox property.The O 2 -TPD results indicate that H 2 activation promoted the migration of oxygen species from deeper layers to the surface.As a result, the concentration of both surface chemisorbed oxygen and surface lattice oxygen increased, potentially providing more active oxygen species for CO oxidation.
In situ DRIFTS experiments were performed to investigate the reaction mechanism.The results indicate that the predominant reaction pathway followed the MvK mechanism at low temperatures (<175 • C).The specific procedure was as follows: (1) Firstly, the CO combined with Cu + to form Cu + -CO species (Equation ( 1)).The H 2 activation increased the quantity of Cu + species, which enhanced the reaction procedure of Equation (1).Subsequently, the Cu + -CO species reacted with surface lattice oxygen to directly generate CO 2 and leave the oxygen vacancies, as displayed in Equation (2).Finally, the oxygen vacancies were resupplemented by gas O 2 , and the cyclic process was completed (Equation ( 3)).
Molecules 2024, 29, x FOR PEER REVIEW 10 of 14 analysis (Figure 2) showed that the pure Fe2O3 had good crystallization and large grain size, and the addition of Cu species can decrease the crystallization.In addition, some peaks attributed to CuO disappeared with the H2 pretreatment at 100 and 150 °C, confirming that the CuO was reduced to Cu2O.After pretreatment at 200 °C, the metal Cu was produced.The specific surface area increased slightly with the introduction of Cu species (Table 1), which could promote the dispersion of active components and adsorption of reaction gas.Following H2 activation, there was minimal change observed in the specific surface area, average pore diameter, and pore volumes of the catalysts.This suggests that the surface physical structure may not be the primary factor contributing to the improvement in catalytic performance.Furthermore, the XPS results (Figure 4) illustrate that the percentage of Cu + species increased on the H2-activated catalyst, which could offer more active sites for CO, and more Oα species were produced with the H2 pretreatment, which proved that more oxygen vacancy was formed.This could facilitate the activation of oxygen species and the circulation of the reaction.In addition, the Fe 2+ species was generated, in agreement with the XRD results, which was beneficial to the dissociation of O2 to oxygen atoms.Moreover, the H2-TPR analysis indicated that the moderate H2 activation could enhance the reducibility of the CuFeOx catalyst, which could increase the catalytic activity for CO oxidation.However, the H2 activation at high temperature led to the decrease in H2 consumption and redox property.The O2-TPD results indicate that H2 activation promoted the migration of oxygen species from deeper layers to the surface.As a result, the concentration of both surface chemisorbed oxygen and surface lattice oxygen increased, potentially providing more active oxygen species for CO oxidation.
In situ DRIFTS experiments were performed to investigate the reaction mechanism.The results indicate that the predominant reaction pathway followed the MvK mechanism at low temperatures (<175 °C).The specific procedure was as follows: (1) Firstly, the CO combined with Cu + to form Cu + -CO species (Equation ( 1)).The H2 activation increased the quantity of Cu + species, which enhanced the reaction procedure of Equation (1).Subsequently, the Cu + -CO species reacted with surface lattice oxygen to directly generate CO2 and leave the oxygen vacancies, as displayed in Equation (2).Finally, the oxygen vacancies were resupplemented by gas O2, and the cyclic process was completed(Equation (3)).
CO oxidation on the CuFeOx catalyst at high temperatures (>175 °C) proceeded via both the MvK and the L-H mechanism, with the latter described in Equations ( 4)- (6).The gas O2 was captured by oxygen vacancy and formed chemisorbed oxygen species, as shown in Equation ( 4).The H2 soft pretreatment increased the concentration of oxygen vacancy on the CuFeOx catalyst, which promoted the process of Equation ( 4).Then, the chemisorbed oxygen species reacted with Cu + -CO species (Equation ( 1)) to generate carbonate species, as exhibited in Equation ( 5).The carbonate species decomposed into (1) Molecules 2024, 29, x FOR PEER REVIEW 10 of 14 analysis (Figure 2) showed that the pure Fe2O3 had good crystallization and large grain size, and the addition of Cu species can decrease the crystallization.In addition, some peaks attributed to CuO disappeared with the H2 pretreatment at 100 and 150 °C, confirming that the CuO was reduced to Cu2O.After pretreatment at 200 °C, the metal Cu was produced.The specific surface area increased slightly with the introduction of Cu species (Table 1), which could promote the dispersion of active components and adsorption of reaction gas.Following H2 activation, there was minimal change observed in the specific surface area, average pore diameter, and pore volumes of the catalysts.This suggests that the surface physical structure may not be the primary factor contributing to the improvement in catalytic performance.Furthermore, the XPS results (Figure 4) illustrate that the percentage of Cu + species increased on the H2-activated catalyst, which could offer more active sites for CO, and more Oα species were produced with the H2 pretreatment, which proved that more oxygen vacancy was formed.This could facilitate the activation of oxygen species and the circulation of the reaction.In addition, the Fe 2+ species was generated, in agreement with the XRD results, which was beneficial to the dissociation of O2 to oxygen atoms.Moreover, the H2-TPR analysis indicated that the moderate H2 activation could enhance the reducibility of the CuFeOx catalyst, which could increase the catalytic activity for CO oxidation.However, the H2 activation at high temperature led to the decrease in H2 consumption and redox property.The O2-TPD results indicate that H2 activation promoted the migration of oxygen species from deeper layers to the surface.As a result, the concentration of both surface chemisorbed oxygen and surface lattice oxygen increased, potentially providing more active oxygen species for CO oxidation.
In situ DRIFTS experiments were performed to investigate the reaction mechanism.The results indicate that the predominant reaction pathway followed the MvK mechanism at low temperatures (<175 °C).The specific procedure was as follows: (1) Firstly, the CO combined with Cu + to form Cu + -CO species (Equation ( 1)).The H2 activation increased the quantity of Cu + species, which enhanced the reaction procedure of Equation (1).Subsequently, the Cu + -CO species reacted with surface lattice oxygen to directly generate CO2 and leave the oxygen vacancies, as displayed in Equation (2).Finally, the oxygen vacancies were resupplemented by gas O2, and the cyclic process was completed(Equation (3)).
CO oxidation on the CuFeOx catalyst at high temperatures (>175 °C) proceeded via both the MvK and the L-H mechanism, with the latter described in Equations ( 4)- (6).The gas O2 was captured by oxygen vacancy and formed chemisorbed oxygen species, as shown in Equation ( 4).The H2 soft pretreatment increased the concentration of oxygen vacancy on the CuFeOx catalyst, which promoted the process of Equation ( 4).Then, the chemisorbed oxygen species reacted with Cu + -CO species (Equation ( 1)) to generate carbonate species, as exhibited in Equation (5).The carbonate species decomposed into CO oxidation on the CuFeO x catalyst at high temperatures (>175 • C) proceeded via both the MvK and the L-H mechanism, with the latter described in Equations ( 4)- (6).The gas O 2 was captured by oxygen vacancy and formed chemisorbed oxygen species, as shown in Equation (4).The H 2 soft pretreatment increased the concentration of oxygen vacancy on the CuFeO x catalyst, which promoted the process of Equation ( 4).Then, the chemisorbed oxygen species reacted with Cu + -CO species (Equation ( 1)) to generate carbonate species, as exhibited in Equation (5).The carbonate species decomposed into CO 2 , as described in Equation (6).The H 2 activation accelerated the decomposition of carbonate species, which alleviated the accumulation of carbonate occupying the active sites and improved the efficiency of CO oxidation.Based on the above, a possible reaction model is proposed and is illustrated in Scheme 1.
Molecules 2024, 29, x FOR PEER REVIEW 11 of 14 CO2, as described in Equation ( 6).The H2 activation accelerated the decomposition of carbonate species, which alleviated the accumulation of carbonate occupying the active sites and improved the efficiency of CO oxidation.Based on the above, a possible reaction model is proposed and is illustrated in Scheme 1.
Scheme 1. Mechanism model of CO oxidation on CuFeOx and CuFeOx-100 catalysts.

Preparation of Catalyst
The catalysts were synthesized using the coimpregnation method.Cu(NO3)2•3H2O and Fe(NO3)3•9H2O were dissolved in ionized water with the molar rate of Cu:Fe = 1:2.Then, the solution was stirred by the magnetic force and heating in a water bath until the water was completely evaporated.Subsequently, the solid was dried for 24 h at 80 °C.Finally, it was calcined at 450 °C for 4 h.The obtained sample was donated as a CuFeOx catalyst.In the H2 activation stage, the tube furnace was vacuumed, and then CuFeOx was heated to 100 °C under a N2 protective atmosphere.Finally, 10% H2/He was pretreated for 0.5 h.The obtained sample was marked as the CuFeOx-100 catalyst, and the samples pretreated with 10% H2/He at 150 and 200 °C were marked as the CuFeOx-150 and CuFeOx-200 catalyst, respectively.CO2, as described in Equation ( 6).The H2 activation accelerated the decomposition of carbonate species, which alleviated the accumulation of carbonate occupying the active sites and improved the efficiency of CO oxidation.Based on the above, a possible reaction model is proposed and is illustrated in Scheme 1.
Scheme 1. Mechanism model of CO oxidation on CuFeOx and CuFeOx-100 catalysts.

Preparation of Catalyst
The catalysts were synthesized using the coimpregnation method.Cu(NO3)2•3H2O and Fe(NO3)3•9H2O were dissolved in ionized water with the molar rate of Cu:Fe = 1:2.Then, the solution was stirred by the magnetic force and heating in a water bath until the water was completely evaporated.Subsequently, the solid was dried for 24 h at 80 °C.Finally, it was calcined at 450 °C for 4 h.The obtained sample was donated as a CuFeOx catalyst.In the H2 activation stage, the tube furnace was vacuumed, and then CuFeOx was heated to 100 °C under a N2 protective atmosphere.Finally, 10% H2/He was pretreated for 0.5 h.The obtained sample was marked as the CuFeOx-100 catalyst, and the samples pretreated with 10% H2/He at 150 and 200 °C were marked as the CuFeOx-150 and CuFeOx-200 catalyst, respectively.CO2, as described in Equation ( 6).The H2 activation accelerated the decomposition of carbonate species, which alleviated the accumulation of carbonate occupying the active sites and improved the efficiency of CO oxidation.Based on the above, a possible reaction model is proposed and is illustrated in Scheme 1.
Scheme 1. Mechanism model of CO oxidation on CuFeOx and CuFeOx-100 catalysts.

Preparation of Catalyst
The catalysts were synthesized using the coimpregnation method.Cu(NO3)2•3H2O and Fe(NO3)3•9H2O were dissolved in ionized water with the molar rate of Cu:Fe = 1:2.Then, the solution was stirred by the magnetic force and heating in a water bath until the water was completely evaporated.Subsequently, the solid was dried for 24 h at 80 °C.Finally, it was calcined at 450 °C for 4 h.The obtained sample was donated as a CuFeOx catalyst.In the H2 activation stage, the tube furnace was vacuumed, and then CuFeOx was heated to 100 °C under a N2 protective atmosphere.Finally, 10% H2/He was pretreated for 0.5 h.The obtained sample was marked as the CuFeOx-100 catalyst, and the samples pretreated with 10% H2/He at 150 and 200 °C were marked as the CuFeOx-150 and CuFeOx-200 catalyst, respectively.CO2, as described in Equation (6).The H2 activation accelerated the decomposition of carbonate species, which alleviated the accumulation of carbonate occupying the active sites and improved the efficiency of CO oxidation.Based on the above, a possible reaction model is proposed and is illustrated in Scheme 1.

Preparation of Catalyst
The catalysts were synthesized using the coimpregnation method.Cu(NO3)2•3H2O and Fe(NO3)3•9H2O were dissolved in ionized water with the molar rate of Cu:Fe = 1:2.Then, the solution was stirred by the magnetic force and heating in a water bath until the water was completely evaporated.Subsequently, the solid was dried for 24 h at 80 °C.Finally, it was calcined at 450 °C for 4 h.The obtained sample was donated as a CuFeOx catalyst.In the H2 activation stage, the tube furnace was vacuumed, and then CuFeOx was heated to 100 °C under a N2 protective atmosphere.Finally, 10% H2/He was pretreated for 0.5 h.The obtained sample was marked as the CuFeOx-100 catalyst, and the samples pretreated with 10% H2/He at 150 and 200 °C were marked as the CuFeOx-150 and CuFeOx-200 catalyst, respectively.

Preparation of Catalyst
The catalysts were synthesized using the coimpregnation method.Cu(NO 3 ) 2 •3H 2 O and Fe(NO 3 ) 3 •9H 2 O were dissolved in ionized water with the molar rate of Cu:Fe = 1:2.Then, the solution was stirred by the magnetic force and heating in a water bath until the water was completely evaporated.Subsequently, the solid was dried for 24 h at 80 • C. Finally, it was calcined at 450 • C for 4 h.The obtained sample was donated as a CuFeO x catalyst.In the H 2 activation stage, the tube furnace was vacuumed, and then CuFeO x was heated to 100 • C under a N 2 protective atmosphere.Finally, 10% H 2 /He was pretreated for 0.5 h.The obtained sample was marked as the CuFeO x -100 catalyst, and the samples pretreated with 10% H 2 /He at 150 and 200 • C were marked as the CuFeO x -150 and CuFeO x -200 catalyst, respectively.

Catalytic Performance Test
The catalytic activity was evaluated on a fixed-bed quartz reactor with an inner diameter of 8 mm, and 300 mg of catalyst was used for each test.A type K thermocouple was inserted into the catalyst bed to monitor the reaction temperature.The composition of the simulated gas mixture was 1% CO, 10% O 2 , and N 2 balance.The total gas flow rate was maintained at 300 mL/min, while the reaction temperature was incrementally increased from 25 to 300 • C at a heating rate of 5 • C/min.Test points were recorded at 25 • C intervals, and the CO concentration at each point remained constant for 30 min.Gas concentrations at the inlet and outlet were measured using an online gas chromatograph.The CO conversion rate was determined by employing Equation (7): where the [CO] in and [CO] out represent the concentration of CO at the inlet and outlet, respectively.

Catalyst Characterization
The physicochemical properties of catalysts were charactered with XRD, BET, XPS, H 2 -TPR, O 2 -TPD, and in situ DRIFTS, and can be found in the supporting information (SI).

Conclusions
In this work, the effect of H 2 activation on the performance of the CuFeO x catalyst for low-temperature CO oxidation was investigated.It was found that the soft H 2 pretreatment could effectively improve the activity of the CuFeO x catalyst, and the catalyst activated at 100 • C displayed the highest performance, which corresponded to 99.6% CO conversion at 175 • C. The influence of H 2 activation on the surface physical structure was negligible.The H 2 activation enhanced the reducibility of CuFeO x catalyst, thus improving the low-temperature activity.In addition, the H 2 activation generated the Fe 2+ species, which was beneficial to the dissociation of O 2 .Furthermore, the amount of O α species and oxygen vacancy increased after H 2 activation, which increased the cycle efficiency of the CO oxidation process.Moreover, the migration of oxygen species from the deep layer to the surface was promoted, which could provide more active oxygen species for CO oxidation.The in situ DRFITS demonstrated that the CO oxidation pathway mainly involved the MvK mechanism at low temperature, and both MvK and L-H mechanisms at high temperature.The Cu + -CO and carbonate species were the main active intermediates, and the H 2 activation increased the amount of Cu + species and accelerated the decomposition of carbonate species, which increased the activity of the CuFeO x catalyst.

Figure 4 .
Figure 4. H 2 -TPR profiles (a) and the amount of H 2 consumption (b) of catalysts.

Figure 5 .
Figure 5. (a) O 2 −TPD profiles and (b) proportion of oxygen species of catalysts.

Scheme 1 .
Scheme 1. Mechanism model of CO oxidation on CuFeO x and CuFeO x -100 catalysts.

Table 1 .
Catalysts' BET specific surface area, pore volume, and average pore size.

Table 1 .
Catalysts' BET specific surface area, pore volume, and average pore size.

Table 2 .
XPS data of the catalysts.